1. Field of the Invention
The present invention relates to an optical amplifying-and-repeating technique for collectively amplifying wavelength division multiplexed (WDM) signal lights transmitted through an optical transmission path. In particular, the invention relates to an optical repeater using Raman amplification and a WDM optical transmission system and the like for Raman amplifying WDM signal lights propagated through a plurality of transmission systems.
2. Description of Related Art
In a conventional WDM optical amplifying-and-repeating transmission system wherein a WDM optical transmission system and an optical amplifying-and-repeating transmission system are combined, it is possible to collectively amplify optical signals having two or more different wavelengths by using optical amplifiers, and also possible to realize high capacity and long distance optical transmission using a simple (economical) structure.
A typical WDM optical amplifying-and-repeating transmission system comprises, for example as shown in FIG. 18, an optical sender system 100 for converting electrical signals into optical signals, a transmission system 110 having a constitution wherein an optical fiber transmission path for transmitting optical signals and an optical amplification repeater for amplifying those optical signals to compensate for losses in the transmission path to send them to a next optical transmission path are repeatedly combined with each other, and an optical receiver system 120 for demodulating the optical signals to electrical signals.
In the above mentioned WDM optical amplifying-and-repeating transmission system, as a conventional WDM optical amplifier to be applied as an optical amplification repeater or the like of the transmission system 110, for example, there is known a method for amplifying WDM signal lights collectively by using an optical fiber amplifier wherein excitation light is supplied to an optical fiber doped with a rare earth element (rare earth element doped fiber amplifier; REDFA). This REDFA has a characteristic that the signal light band is limited by the optical amplification band to be determined depending on the doped element.
Alternatively, a Raman amplifier utilizing simulated Raman scattering can amplify a wider signal light wavelength band than the REDFA, and there is a prospect of application to future optical amplification repeaters. However, since the abovementioned Raman amplifier requires comparatively greater excitation light power than the REDFA, for example when applied to a system in which repeater power consumption is limited, such as an undersea cable system and the like, it is important to supply excitation light to the Raman amplifier effectively.
The construction of a conventional Raman amplifier is, for example, disclosed in Japanese Unexamined Patent Publication No. 2000-98433. In this conventional Raman amplifier, excitation lights of required wavelengths, output from a plurality of excitation light sources that are constituted by using semiconductor lasers of Fabry-Perot type or the like, are multiplexed to be supplied to the Raman amplification media.
Incidentally, for optical amplification repeaters used in WDM optical amplifying-and-repeating transmission systems, it is required to not only amplify a WDM signal light transmitted in one direction, but also to amplify a WDM signal light propagated through other transmission path for increasing transmission capacity, or to amplify WDM signal lights transmitted in both the ascending direction and descending direction. The realization of such a requirement is especially important for undersea cable systems and the like.
Prior art In which an optical amplification repeater that can amplify WDM signal lights in both ascending and descending directions is realized using a REDFA is described in, for example, Japanese Unexamined Patent Publications No. 5-268166, No. 6-314833, No. 7-176813, No. 8-54580, No. 8-304860, No. 2000-49405 and No. 2000-114625 and the like.
However, at the present time, a technique to realize an optical amplification repeater coping with a plurality of transmission systems by using the Raman amplifier has not been realized. The Raman amplifier can amplify a wider signal light wavelength band than the REDFA as mentioned before, and it is highly usable to realize an optical amplification repeater that will cope with a plurality of transmission systems.
If an optical amplification repeater coping with a plurality of transmission systems is to be realized by applying a conventional Raman amplifier as described above, since the conventional structure is for amplifying a WDM signal light propagated through one optical fiber (one transmission system), individual Raman amplifiers must be installed corresponding to each transmission system. Therefore, it is necessary to drive a number of excitation light sources for Raman amplification. However, such a structure of optical amplification repeater is not realistic for an undersea cable system and the like in which power consumption is limited, and hence the problem of improving excitation efficiency needs to be solved.
Furthermore, in a case where WDM signal lights of a plurality of transmission systems are Raman amplified by one optical amplification repeater, if Raman amplification operation is not controlled such that in each transmission system, important parameters, such as total output light power of the repeater, optical power (or gain) in signal light wavelength bands corresponding to excitation light wavelengths, optical power (or gain) deviations and the like, are all within allowable ranges, there is a possibility that WDM signal lights of all of the transmission systems cannot be transmitted under required error rates, which is a problem.
The present invention addresses the abovementioned points, with the object of providing a technique related to optical repeaters using Raman amplification and the like that enables Raman amplification of WDM signal lights that are propagated through a plurality of transmission systems, with high excitation efficiency and stability.
To achieve the abovementioned object, an optical repeater using Raman amplification according to the present invention, which supplies excitation light to a Raman amplification medium of each one of a plurality of transmission systems transmitting WDM signal lights, and Raman amplifies each WDM signal light transmitted through each of the transmission systems, comprises: a plurality of excitation light generation sections for generating excitation lights of different wavelengths corresponding to the wavelength bands of the WDM signal lights; an optical multiplexing/demultiplexing section having a plurality of input ports corresponding to the respective excitation light generation sections, and a plurality of output ports corresponding to the respective transmission systems, for multiplexing excitation lights input to the input ports from the excitation light generation sections, and demultiplexing the multiplexed excitation light for output from the output ports; and a plurality of excitation light multiplexing sections for supplying excitation lights output from the respective output ports of the optical multiplexing/demultiplexing section to the Raman amplification media of the respective transmission systems, respectively.
In such a construction, excitation lights of different wavelengths generated in the plurality of excitation light generation sections are multiplexed and then demultiplexed corresponding to the number of the plurality of transmission systems in the optical multiplexing/demultiplexing section and then supplied to the Raman amplification media of the respective transmission systems, and the WDM signal light propagated through each Raman amplification medium is Raman amplified. As a result, a plurality of excitation lights are supplied to the Raman amplification media of the respective transmission systems with no loss and in balance, so that WDM signal lights propagated through a plurality of transmission systems can be Raman amplified with high excitation efficiency, thus enabling the realization of a small-sized optical repeater using Raman amplification with low power consumption.
Furthermore, it is preferable that, the abovementioned optical repeater using Raman amplification is provided with signal light power detecting section for detecting WDM signal light powers after Raman amplification in the respective transmission systems, and a control section for controlling the excitation light power output from each of the excitation light generation sections based on the detection result of the signal light power detecting section.
According to such a construction, based on the WDM signal light power after Raman amplification in each transmission system, the power of each excitation light to be transmitted to the optical multiplexing/demultiplexing section is feedback controlled. As a result, an influence on Raman amplification operation by the dispersion of the excitation light power output from the optical multiplexing/demultiplexing section and the characteristic variation of optical transmission paths and the like can be reduced, thus enabling stable Raman amplification of respective WDM signal lights propagated through a plurality of transmission systems.
A specific structure of the abovementioned optic repeater, using Raman amplification may be such that the signal light power detecting section detects the total optical power of WDM signal light after Raman amplification in each of the transmission systems, and the control section is provided with a mean value computation section for computing a mean value of the total optical power of each transmission system, which is detected in the signal light power detecting section, a comparison section for obtaining a difference between a preset target mean value and the mean value of the total optical power computed in the mean value computation section; and an excitation light power control section for controlling the excitation light power output from each of the excitation generation sections according to the difference value obtained in the comparison section. Furthermore, it is preferable that the abovementioned control section has a correction section for performing weighting correction on the difference value obtained in the comparison section corresponding to each of the excitation light generation sections, and that the excitation light power control section controls the excitation light power output from each of the excitation light generation sections according to the value weighting corrected in the correction section.
According to such a construction, the power of each excitation light is feedback controlled such that the mean value of the total optical power of the WDM signal lights after Raman amplification in the respective transmission systems approaches a target value set in advance. Furthermore, if a correction section is installed to perform weighting correction corresponding to each excitation light generation section, it is possible to control the excitation light power with high accuracy taking into consideration a change in Raman gain different for each wavelength of each excitation light.
Another specific structure of the abovementioned optical repeater using Raman amplification may be such that the signal light power detecting section divides the wavelength division multiplexed signal light after Raman amplification in each of said transmission systems into gain wavelength bands corresponding to the excitation light wavelength of each of said excitation light generation sections, and detects the optical powers of said signal lights for each of the gain wavelength bands and the control section is provided with a mean value computation section for gathering the optical power for each of the gain wavelength bands in each transmission system detected in the signal light power detecting section and computing a mean value, a comparison section for obtaining a difference between a target mean value set in advance corresponding to the gain wavelength band and the mean value of the optical power for each of the gain wavelength bands computed in the mean value computation section, and an excitation light power control section for controlling the excitation light power output from each of the excitation light generation sections according to the difference value for each of the gain wavelength bands, obtained in the comparison section.
According to such a construction, the WDM signal light power after Raman amplification in each transmission system is monitored for each gain wavelength band corresponding to each of the excitation light wavelengths, to obtain the mean value, and the power of each excitation light is feedback controlled such that the mean value of each gain wavelength band approaches the target value.
Another specific structure of the abovementioned optical repeater using Raman amplification may be such that the signal light power detecting section divides the wavelength division multiplexed signal light after Raman amplification in each of said transmission systems into gain wavelength bands corresponding to the excitation light wavelength of each of said excitation light generation sections, and detects optical powers of said signal lights for each of the gain wavelength bands, and the control section is provided with a comparison section for obtaining, for the optical power of each transmission system detected in the signal light power detecting section, differences of the optical powers of the other gain wavelength bands to the optical power of a reference gain wavelength band set in advance, a mean value computation section for gathering difference values obtained by the comparison section for each of the gain wavelength bands and computing a mean value, and an excitation light power control section for controlling the excitation light power output from each of the excitation light generation sections according to the mean value of the differences of the respective gain wavelength bands, computed in the mean value computation section.
According to such a construction, the WDM signal light power after Raman amplification in each transmission system is monitored for each gain wavelength band corresponding to each of the excitation light wavelengths, and differences of the optical powers of the other gain wavelength bands to the optical power of the reference gain wavelength band are obtained. Then, the mean value of the differences in the respective transmission systems is computed for each gain wavelength band, and the power of each excitation light is feedback controlled based on the computation result. As a result, it is also possible to compensate for deterioration of characteristics with age.
The abovementioned optical repeater using Raman amplification may be such that the signal light power detecting section selects any one of the plurality of transmission systems and detects the power of the WDM signal light after Raman amplification, and the control section controls the power of the excitation light output from each of the excitation light generation sections based only on the detection result for the transmission system selected in the signal light power detecting section. According to such a construction, the feedback control of the excitation light power corresponding to the gain wavelength band is performed only for the transmission system selected from among a plurality of transmission systems.
The optical repeater using Raman amplification as described above has a plurality of transmission systems for transmitting WDM signal lights between transmitting terminals and receiving terminals, and in a WDM optical transmission system provided with an optical repeater on the optical transmission path of each transmission system, is suitable as an optical repeater.
Other objects, features and advantages of this invention will become apparent in the following description of embodiments in relation to the accompanying drawings.